A heat spreader is attached to a semiconductor element via a heat dissipation material (a thermal interface material: TIM). As the heat dissipation material, indium may be exemplified. However, since the heat generated by a semiconductor element used for high performance computing (HPC) is very high, a material having a higher heat resistance and a higher thermal conductivity is required. As such a material, a carbon nanotube sheet may be exemplified. A carbon nanotube sheet is composed of a bundle of numerous carbon nanotubes bonded to each other by a van der Waals force.
However, the van der Waals force is significantly weaker than a metallic bonding force. Thus, at the time of handling, the bundle of carbon nanotubes may be easily unraveled, and the sheet shape thereof is difficult to maintain. Although there has been proposed a carbon nanotube sheet in which the bonding of carbon nanotubes is reinforced by a resin, the thermal conductivity is reduced. The thermal conductivity of the resin ranges from about 1 W/m·K to about 10 W/m·K, which is equal to or less than about 1/100 of the thermal conductivity of carbon nanotubes.
Although there has also been proposed a carbon nanotube sheet in which tip ends of carbon nanotubes are bound with a polymer film, a sufficient thermal conductivity is not obtained even with this carbon nanotube sheet. When carbon nanotubes are sandwiched between a heat spreader and a semiconductor element, a large stress may act on the carbon nanotubes so that the carbon nanotubes may buckle.
The followings are reference documents.    [Document 1] Japanese Laid-Open Patent Publication No. 2005-150362,    [Document 2] Japanese Laid-Open Patent Publication No. 2006-147801,    [Document 3] Japanese Laid-Open Patent Publication No. 2014-003144 and    [Document 4] Japanese Laid-Open Patent Publication No. 2010-121867.